Bacterial levan (BL) induces an immune response in mice in which a large fraction of antibodies express a cross-reactive idiotype shared with inulin-binding myeloma proteins. The percentage of anti-BL antibodies expressing the cross-reactive idiotype decreases with time after immunization, whereas the magnitude of the auto-anti-idiotypic response increases. This suggests that auto-anti-idiotypic antibody can suppress the complimentary anti-BL response. Athymic mice are not able to mount an auto-anti-idiotypic response; but, they produce an antibody response to BL that shows a cyclic (oscillatory) pattern. This indicates that auto-anti-idiotypic antibodies are not the main element in causing oscillations. However, they do modulate the expression of cross-reactive idiotypes in euthymic mice and there is no relationship between the suppressive property of auto-anti-idiotypic antibodies and the induction or maintenance of tolerance.